Elemental phosphorus was produced by the Tennessee Valley Authority (TVA) at Muscle Shoals, Alabama, by a process consisting of the following steps.
1. Phosphate ore was partially dried. PA0 2. Partially dried ore was agglomerated, and the agglomerates were indurated by heating. PA0 3. Indurated agglomerates were used as feedstock for production of elemental phosphorus. Other furnace feed materials were metallurgical coke and silica rock. PA0 4. Mixture of agglomerated phosphate ore, metallurgical coke, and silica rock was smelted in electric furnaces. PA0 5. Gases evolved from electric furnaces and particulates in gases were removed from said gases by treating in electrostatic precipitators. Particulates collected in this manner are a waste called precipitator dust. PA0 6. Cleaned gases were cooled to condense elemental phosphorus. PA0 0.06 ton per ton of elemental phosphorus produced. Analysis of the precipitator dust on a dry basis is given in table 1. PA0 1. Incineration in a facility approved by the Environmental Protection Agency (EPA). PA0 2. Disposal of the residue in a secure landfill as stipulated by the 1984 Amendments to the Resource Conservation and Recovery Act. PA0 1. An acidic phosphorus compound is reacted with an alkaline substance to prepare a binder to agglomerate precipitator dust. The preferred acidic phosphorus compound is phosphoric acid and the preferred alkaline substance is ground phosphate ore. Phosphoric acid and ground phosphate ore are combined in proportions required for the preparation of monocalcium phosphate. PA0 2. Precipitator dust is tumbled with a slurry formed by mixing the acidic phosphorus compound with the alkaline substance. Reaction between the two materials begins in the reactor and continues in the tumbler. Agglomerates are formed by tumbling action wherein particles are aggregated by a combination of surface tension forces and bridging by a salt formed when an acid is neutralized with an alkaline substance. When the preferred reacting materials--phosphoric acid and ground phosphate ore--are used, the salt will be monocalcium phosphate monohydrate. PA0 3. Agglomerates formed in 2 above are hardened by heating them in an indurator. Reaction between phosphoric acid and ground phosphate ore is driven to completion by heating. Agglomerates are desiccated and discrete particles are held together by salt bridges. A temperature of about 220.degree. F. drives the reaction between phosphoric acid and ground phosphate ore to completion. Also, water of hydration in monocalcium phosphate monohydrate is volatilized by heating to 220.degree. F. An induration temperature of 220.degree. F. is taken as the lower limit. Precipitator dust may contain carbon carried over from the electric furnace and the carbon will reduce calcium phosphate beginning at about 1000.degree. C., or 1832.degree. F. The upper temperature limit for induration is taken to be about 1800.degree. F. Therefore, temperature limits for induration are 220.degree. to 1832.degree. F., but the preferred temperature range is 550.degree. to 600.degree. F. PA0 4. Gases emitted from reactor, tumbler, and indurator are contacted with an aqueous scrubbing medium to condense elemental phosphorus, to absorb fluorine compounds, and to collect entrained particulates. When the pH is in the range of 5.5 to 6.0, or higher, fluorine compounds are readily absorbed. But pH values greater than about 6.0 will result in large ammonia losses in the stack gas. PA0 5. The scrubbing medium is recirculated as described above. Elemental phosphorus, fluorine compounds, and particulates accumulate as a result of recirculation. A stream of the recirculating scrubbing medium is bled off and replaced with fresh water to prevent excessive concentrations of elemental phosphorus, fluorine compounds, and particulates. The bleedoff is used as feedstock for the production of suspension fertilizer wherein water is required in the manufacturing process. Thus water required for production of the fertilizer is derived from water added to the scrubbing medium in step 4. Elemental phosphorus in the bleedoff is oxidized to form P.sub.2 O.sub.5 which is hydrated to phosphoric acid, and the phosphoric acid is neutralized with ammonia to form ammonium phosphate. PA0 6. Hardened agglomerates from step 3 are smelted in a submerged-arc electric furnace to produce elemental phosphorus.
The quantity of precipitator dust collected was about
TABLE 1 ______________________________________ Composition of Precipitator dust Percent, dry basis ______________________________________ P.sub.2 O.sub.5 27.7 CaO 13.8 SiO.sub.2 17.3 Fe.sub.2 O.sub.3 1.7 Al.sub.2 O.sub.3 3.6 F 6.3 K.sub.2 O 17.4 MgO 0.9 MnO.sub.2 0.1 Na.sub.2 O 3.1 S 0.1 Total 92.0 ______________________________________
In addition to constituents shown in table 1, precipitator dust may contain up to 1.2 percent elemental phosphorus, depending on the temperature of dust as it is discharged from the electrostatic precipitator. When the dust temperature is 900.degree. F., or higher, the precipitator dust will contain essentially no elemental phosphorus. However, the gas temperature at the precipitator inlet is normally in the range of 650.degree. to 700.degree. F. and the average elemental phosphorus content of the dust is in the range of 0.1 to 0.3 percent.
Precipitator dust has potential value as a fertilizer because it contains relatively high percentages of P.sub.2 O.sub.5 and K.sub.2 O, both of which are nutrients. An endeavor was made to convert the waste into granular fertilizer by oxidizing the elemental phosphorus with air, granulating the phosphorus-free material, bagging the granules, and distributing the material on agricultural land.
Unfortunately, elemental phosphorus was not readily oxidized. Since the element ignites spontaneously, its presence in a fertilizer is a safety hazard. Paper bags containing granulated precipitator dust ignited, and it was evident the presence of unoxidized elemental phosphorus was a fire hazard. The project to recover precipitator dust as granular fertilizer was abandoned at TVA.
Large-scale production of elemental phosphorus began at TVA in 1934 at a fertilizer research facility now called the National Fertilizer Development Center (NFDC). The first electrostatic precipitator was installed on a phosphorus production unit in 1939 and precipitators were subsequently installed on all the production units. TVA discontinued production of elemental phosphorus in 1976 and production units have been dismantled.
All precipitator dust produced at NFDC is in storage piles exposed to the weather except for that recovered as fertilizer. It is estimated 17,000 tons is stored at NFDC. Since elemental phosphorus is an acutely toxic chemical, and runoff from precipitation is a potential pollution problem, continued storage of the waste in outside piles does not appear to comply with the 1984 Amendments to the Resource Conservation and Recovery Act (public law 99-499).
Heretofore the only means of disposing of the phosphorus-containing precipitator dust would be the following two-step process.
Disposal of the waste by the two-step process will be costly. Furthermore, placement in a landfill is the least desirable option of the various methods for waste disposal. A waste minimization process was sought which would be less costly than incineration and landfilling. Also, a waste minimization method such as recovery or recycle is preferred over incineration or landfilling.